Kinematic mounts, otherwise known as kinematic couplings or constraints, are commonly used to couple measuring equipment or instruments to a base or substructure and for coupling rigid parts to hard mating surfaces, where despite repeated disassembly and reassembly the plates remain in the same relative position to one another as when previously assembled.
Three linear motions or translations (X, Y, Z) and three angular motions or rotations (θx, θy, θz) are necessary to describe fully the motion and position of a solid body in space. For a rigid body to be completely fixed in space, despite repeated disassembly and reassembly, all six degrees of freedom need to be constrained. In other words, the three translations and the three rotations must be constrained with respect to some arbitrary fixed coordinate system. A mount is kinematic when all six degrees of freedom are constrained, in which any additional constraints would be redundant. Accordingly, a kinematic mount has six independent constraints.
Simple kinematic mounts use a cone or ball, a groove, and a flat in the front plate of the mount to constrain unwanted motion. The ball constrains motion in the X, Y, and Z axes. The groove constrains motion θy (pitch) and θx (roll). The flat constrains motion in θz (yaw). The chief disadvantage of cone/ball mounts is that some cross-coupling occurs between the translational and rotational axes because the rotational axes are not centered on the surface on the mounted part, which can cause significant translational errors.
A well-known kinematic mount incorporates a fixed base plate with three V-shaped grooves each forming an angle of approximately 120 degrees with each other groove. The walls of each groove form angles of approximately 45 degrees with the surface of the base plate. On a second plate, three convex spherical members are secured roughly in an equilateral triangular array. When the second plate is rested upon the first plate, each of the three convex spherical members rests within one of the three grooves, contacting the two side walls of each respective second plate, which may be lifted from the base plate and, when replaced, will occupy the identical position relative to the base, which normally remains fixed.
However, the above-described point contacts between each spherical member and groove leads to concentrated forces at these contact points. These concentrated forces lead to high stresses, know as Hertzian stresses, both at the spherical member and at the groove. Accordingly, while this prior art mount is sufficient for light loads, such as laboratory applications or light-duty field applications, it fails in heavy-duty applications, such as when used in space launch vehicles, bridges, buildings, and superstructures, where high intensity vibrations and shocks cause failure at the contact points.
Some kinematic mounts, which are often referred to as isolators, are designed to allow for a transient number of freedom changes (i.e., tilt and magnitude changes), and are used for motion isolation. The transient number of freedom changes is, in some instances, temporary. Typical tilts are those encountered by anchored marine oil rigs and rocket launchers. Typical magnitude changes are those encountered by the seismic base movement of celestial observation telescopes, mirrors, lens, and antennas.
Many skilled artisans have attempted to improve on rigid kinematic mounts by incorporating compliant features. For instance, one such mount incorporates constrained elastomer layers with the V-struts of a three-legged platform support employing ball-in-cone bearings. However, this arrangement is not capable of coping with transient tilts or magnitude variations unless these disturbances are microscopic or marginal. Thus, use of this mount is, unfortunately, limited to optical and semiconductor applications. Other efforts to introduce selected compliance in otherwise rigid kinematic mounts have also been attempted, with marginal or unacceptable results.
And so the current trend in the state of art of kinematic constraint mounting devices is to add compliance damping capabilities in order to balance transitive accelerations and to provide automatic re-centering, all without active control in the shortest time practicable and without imposing additional demands on otherwise used active control systems over the mount and without the need to reboot or reset such systems after the passing of acceleration disturbances. Some applications, such as the blast protection or terrorist hardening of control and command platforms and data centers, cannot afford to reboot and reset when the continuous operation of such systems is vital. The same holds for seismic protection of data, communications, and command centers. The continuous operability of mounted control devices in artificial gravity or induced centripetal fields in space stations is a rather obvious requirement in light of the fact that transients of such field are inevitable and may be frequent.